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Abstract:

A method for producing a nonaqueous electrolyte secondary battery
including a positive electrode containing a positive electrode active
material, a negative electrode containing a negative electrode active
material, and a nonaqueous electrolyte, the negative electrode active
material containing a carbon material and particles of at least one metal
selected from zinc and aluminum. The method includes a step of preparing
an aqueous negative electrode mixture slurry that contains the metal
particles, the carbon material, and a polysaccharide polymer as a
thickener and that has pH adjusted in the range of 6.0 to 9.0; and a step
of forming a negative electrode by applying the negative electrode
mixture slurry to a negative electrode current collector.

Claims:

1. A method for producing a nonaqueous electrolyte secondary battery
including a positive electrode containing a positive electrode active
material, a negative electrode containing a negative electrode active
material, and a nonaqueous electrolyte, the negative electrode active
material containing a carbon material and particles of at least one metal
selected from the group consisting of zinc and aluminum, the method
comprising: a step of preparing an aqueous negative electrode mixture
slurry which contains the metal particles, the carbon material, and a
polysaccharide polymer as a thickener and which has a pH adjusted in the
range of 6.0 to 9.0; and a step of forming the negative electrode by
applying the negative electrode mixture slurry to a negative electrode
current collector.

2. The method for producing a nonaqueous electrolyte secondary battery
according to claim 1, wherein the pH is adjusted in the range of 6.0 to
9.0 by adding a pH buffer component to the negative electrode mixture
slurry.

3. The method for producing a nonaqueous electrolyte secondary battery
according to claim 2, wherein the negative electrode mixture slurry
containing the polysaccharide polymer contains the pH buffer component
before the metal particles are added.

4. The method for producing a nonaqueous electrolyte secondary battery
according to claim 2, wherein the pH buffer component is a phosphate
buffer component.

5. The method for producing a nonaqueous electrolyte secondary battery
according to claim 3, wherein the pH buffer component is a phosphate
buffer component.

8. The method for producing a nonaqueous electrolyte secondary battery
according to claim 1, wherein the polysaccharide polymer is a
carboxymethyl cellulose compound.

9. The method for producing a nonaqueous electrolyte secondary battery
according to claim 2, wherein the polysaccharide polymer is a
carboxymethyl cellulose compound.

10. The method for producing a nonaqueous electrolyte secondary battery
according to claim 3, wherein the polysaccharide polymer is a
carboxymethyl cellulose compound.

11. The method for producing a nonaqueous electrolyte secondary battery
according to claim 4, wherein the polysaccharide polymer is a
carboxymethyl cellulose compound.

12. The method for producing a nonaqueous electrolyte secondary battery
according to claim 5, wherein the polysaccharide polymer is a
carboxymethyl cellulose compound.

13. The method for producing a nonaqueous electrolyte secondary battery
according to claim 6, wherein the polysaccharide polymer is a
carboxymethyl cellulose compound.

14. The method for producing a nonaqueous electrolyte secondary battery
according to claim 2, wherein the average particle diameter of the metal
particles is in the range of 0.5 μm to 50 μm.

15. The method for producing a nonaqueous electrolyte secondary battery
according to claim 3, wherein the average particle diameter of the metal
particles is in the range of 0.5 μm to 50 μm.

16. The method for producing a nonaqueous electrolyte secondary battery
according to claim 4, wherein the average particle diameter of the metal
particles is in the range of 0.5 μm to 50 μm.

17. The method for producing a nonaqueous electrolyte secondary battery
according to claim 12, wherein the average particle diameter of the metal
particles is in the range of 0.5 μm to 50 μm.

18. The method for producing a nonaqueous electrolyte secondary battery
according to claim 6, wherein the average particle diameter of the metal
particles is in the range of 0.5 μm to 50 μm.

19. The method for producing a nonaqueous electrolyte secondary battery
according to claim 1, wherein the metal particles are formed by an
atomization method.

20. A nonaqueous electrolyte secondary battery comprising: a positive
electrode containing a positive electrode active material; a negative
electrode containing a negative electrode active material; and a
nonaqueous electrolyte, wherein the negative electrode includes a
negative electrode active material layer provided on a negative electrode
current collector, and the negative electrode active material layer
contains particles of at least one metal selected from the group
consisting of zinc and aluminum, a carbon material, a polysaccharide
polymer, and a pH buffer component.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present invention claims priority to Japanese Patent
Application No. 2010-221678 filed in the Japan Patent Office on Sep. 30,
2010, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for producing a
nonaqueous electrolyte secondary battery using an aqueous negative
electrode mixture slurry containing particles of at least one metal
selected from the group consisting of zinc and aluminum, and a nonaqueous
electrolyte secondary battery.

[0004] 2. Description of Related Art

[0005] In recent years, nonaqueous electrolyte secondary batteries in
which charge and discharge are performed by moving lithium ions between a
positive electrode and a negative electrode have been used as power
supplies for mobile electronic devices.

[0006] Also, in recent years, reduction in size and weight of mobile
devices such as mobile phones, notebook-size personal computers, PDA
(Personal Digital Assistant), etc. has significantly advanced, and power
consumption has been increased with the addition of multifunctions. In
addition, there has been an increasing demand for nonaqueous electrolyte
secondary batteries used as power supplies of these devices to have a
high capacity and high energy density.

[0007] In the nonaqueous electrolyte secondary batteries, lithium
cobaltate LiCoO2, spinel lithium manganate LiMn2O4, a
lithium-cobalt-nickel-manganese composite oxide, a
lithium-aluminum-nickel-manganese composite oxide, and a
lithium-aluminum-nickel-cobalt composite oxide are known as positive
electrode active materials for positive electrodes. In addition, metallic
lithium, carbon such as graphite, and materials which alloy with lithium,
such as silicon and tin as described in the Journal of Electrochemical
Society 150 (2003) A679 (Non-Patent Document 1) are known as negative
electrode active materials for negative electrodes.

[0008] When metallic lithium is used as a negative electrode active
material, it is difficult to handle and the formation of needle-shaped
dendrites composed of metallic lithium occurs by charge and discharge,
thereby causing internal short-circuit between the negative electrode and
a positive electrode. Therefore, there are problems with battery life,
safety, etc.

[0009] When a carbon material is used as a negative electrode active
material, dendrites do not occur. In particular, use of graphite among
carbon materials has the advantages of excellent chemical durability and
structural stability, a high capacity per unit mass, high reversibility
of lithium occlusion/release reaction, a low action potential, and
excellent flatness. Therefore, graphite is often used for power supplies
of mobile devices.

[0010] However, graphite has the problem that the theoretical capacity of
intercalation complex LiC6 is 372 mAh/g. Thus, it is impossible to
sufficiently comply with the above-described demand for a high capacity
and high energy density.

[0011] In order to produce a nonaqueous electrolyte secondary battery
having a high capacity and high energy density using graphite, a negative
electrode mixture containing graphite having a scaly primary particle
shape is strongly compressed and bonded to a current collector to
increase the packing density of the negative electrode mixture, thereby
increasing the volume specific capacity of the nonaqueous electrolyte
secondary battery.

[0012] However, in this case, when the packing density is increased by
compressing the negative electrode mixture containing graphite, the
graphite having a scaly primary particle shape is excessively oriented
during compression, thereby causing the problems of decreasing the ionic
diffusion rate in the negative electrode mixture to decrease the
discharge capacity and increasing the action potential during discharge
to decrease the energy density.

[0013] In addition, Si or a Si alloy has recently been proposed as a
negative electrode active material having a high capacity density and
high energy density in terms of mass ratio. Such a material exhibits a
high specific capacity per unit mass of 4198 mAh/g in terms of Si.
However, the material has the problem that the action potential at the
time of discharge is higher than that of a graphite negative electrode,
and volumetric expansion/contraction occurs during charge and discharge,
resulting in deterioration in cycling characteristics.

[0014] Besides the above-described silicon (Si), zinc (Zn) and aluminum
(Al) are known as elements that alloy with lithium to exhibit a high
charge/discharge capacity. The theoretical capacity densities of zinc and
aluminum are 410 mAh/g and 993 mAh/g, respectively, and are lower than
the theoretical capacity density of silicon.

[0015] The inventors have found that when a packing density of a negative
electrode mixture is increased by compressing it, a high charge/discharge
capacity and good cycling characteristics can be achieved by using, as a
negative electrode active material, a carbon material, such as graphite,
in combination with zinc or aluminum that shows smaller volumetric
expansion/contraction than silicon during charge/discharge. A technique
of combining a carbon material and an element that alloys with lithium is
disclosed in Japanese Published Unexamined Patent Application Nos.
2004-213927 (Patent Document 1) and 2000-113877 (Patent Document 2).

[0016] Patent Document 1 discloses the use of a negative electrode
material containing a carbonaceous material, a graphite material, and
metal nano fine particles having an average particle diameter of 10 nm or
more and 200 nm or less and composed of a metal element selected from Ag,
Zn, Al, Ga, In, Si, Ge, Sn, and Pb.

[0017] Patent Document 1 also discloses that by using the metal nano fine
particles having a very small average particle diameter from the
beginning, the influence of reduction in size of the particles due to
expansion/contraction accompanying charge and discharge is suppressed,
thereby improving cycling characteristics.

[0018] Patent Document 2 discloses the use of a mixture of graphite and a
conductive aid containing carbon particles which hold a metal that forms
an alloy with lithium. Also, Patent Document 2 discloses that the carbon
particles which hold the metal particles have a smaller particle diameter
than the graphite.

[0019] However, Patent Documents 1 and 2 use an organic solvent-based
slurry and do not disclose a problem with use of an aqueous slurry and do
not disclose a method for resolving the problem.

BRIEF SUMMARY OF THE INVENTION

[0020] An object of the present invention is to provide a method for
producing a nonaqueous electrolyte secondary battery by forming a
negative electrode using an aqueous negative electrode mixture slurry
which contains particles of at least one metal selected from the group
consisting of zinc and aluminum, the method being capable of suppressing
the occurrence of aggregates when the negative electrode is formed. An
object of the present invention is also to provide a nonaqueous
electrolyte secondary battery.

[0021] The present invention provides a method for producing a nonaqueous
electrolyte secondary battery including a positive electrode containing a
positive electrode active material, a negative electrode containing a
negative electrode active material, and a nonaqueous electrolyte, the
negative electrode active material containing a carbon material and
particles of at least one metal selected from the group consisting of
zinc and aluminum. The method includes a step of preparing an aqueous
negative electrode mixture slurry which contains the metal particles, a
carbon material, and a polysaccharide polymer as a thickener and which
has pH adjusted in a range of 6.0 to 9.0, and a step of forming the
negative electrode by applying the negative electrode mixture slurry to a
negative electrode current collector.

[0022] According to an embodiment of the production method according to
the present invention, it is possible to suppress the occurrence of
aggregates when a negative electrode is formed and to produce a
nonaqueous electrolyte secondary battery having a high capacity, a high
energy density, and excellent charge/discharge cycling characteristics.

[0023] According to the present invention, the pH is preferably adjusted
in the range of 6.0 to 9.0 by adding a pH buffer component to the
negative electrode mixture slurry.

[0025] As the pH buffer component, a phosphate buffer component, for
example, a buffer component containing potassium dihydrogen phosphate,
can be used.

[0026] In an embodiment of the present invention, the polysaccharide
polymer used as the thickener is, for example, a carboxymethylcellulose
compound.

[0027] In the present invention, the average particle diameter of the
metal particles is preferably in the range of 0.5 μm to 50 μm.

[0028] The metal particles are preferably formed by an atomization method.

[0029] A nonaqueous electrolyte secondary battery according to the present
invention includes a positive electrode containing a positive electrode
active material, a negative electrode containing a negative electrode
active material, and a nonaqueous electrolyte. The negative electrode
includes a negative electrode active material layer provided on a
negative electrode current collector, the negative electrode active
material layer containing particles of at least one metal selected from
zinc and aluminum, a carbon material, a polysaccharide polymer, and a pH
buffer component.

[0030] According to an embodiment of the present invention, in a method
for producing a nonaqueous electrolyte secondary battery by forming a
negative electrode using an aqueous negative electrode mixture slurry
that contains particles of at least one metal selected from zinc and
aluminum, it is possible to suppress the occurrence of aggregates when
the negative electrode is formed. Therefore, a nonaqueous electrolyte
secondary battery having a high capacity, a high energy density, and
excellent charge/discharge cycling characteristics can be produced.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0031] FIG. 1 is a drawing showing a 10,000-times magnified SEM (scanning
electron microscope) image of zinc particles used in an example according
to the present invention;

[0032] FIG. 2 is a schematic sectional view showing a test cell formed in
an example according to the present invention;

[0033] FIG. 3 is a drawing showing a 5,000-times magnified SEM image of a
surface of a negative electrode formed in Example 1 according to the
present invention;

[0034] FIG. 4 is a drawing showing a 5,000-times magnified SEM reflection
electron image of a surface of a negative electrode formed in Example 1
according to the present invention;

[0035] FIG. 5 is a drawing showing a 5,000-times magnified SEM image of a
surface of a negative electrode formed in Example 2 according to the
present invention;

[0036] FIG. 6 is a drawing showing a 5,000-times magnified SEM reflection
electron image of a surface of a negative electrode formed in Example 2
according to the present invention;

[0037]FIG. 7 is a drawing showing a 5,000-times magnified SEM image of a
surface of a negative electrode formed in Comparative Example 1 according
to the present invention; and

[0038] FIG. 8 is a drawing showing a 5,000-times magnified SEMI reflection
electron image of a surface of a negative electrode formed in Comparative
Example 1.

DETAILED DESCRIPTION OF THE INVENTION

[0039] The present invention is described in further detail below.

[Preparation of Negative Electrode Mixture Slurry]

[0040] A negative electrode mixture slurry of the present invention is an
aqueous slurry having pH adjusted in the range of 6.0 to 9.0 and
containing metal particles, a carbon material, and a polysaccharide
polymer serving as a thickener.

[0041] The metal particles, the carbon material, and the polysaccharide
polymer are described below.

<Metal Particles>

[0042] The metal particles used in the present invention are composed of
at least one metal selected from the group consisting of zinc and
aluminum.

[0043] The average particle diameter of the metal particles is preferably
in the range of 0.5 μm to 50 μm and more preferably in the range of
1 μm to 20 μm.

[0044] Zinc and aluminum have a higher ionization tendency than hydrogen.
Therefore, with a small average particle diameter, it is difficult to
produce the metal particles and the specific surface area is increased.
As a result, the surface may be easily oxidized in air, thereby failing
to achieve sufficient battery characteristics due to inactivation of the
metal.

[0045] On the other hand, with an excessively large average particle
diameter, the metal particles are settled when the negative electrode
mixture slurry is formed, and thus the metal particles are not uniformly
dispersed in the negative electrode mixture. As a result, the effect of
mixing of the metal particles with the carbon material may not be
sufficiently obtained.

[0046] The metal particles used in the present invention are preferably
formed by an atomization method. The atomization method makes easy
control of the average particle diameter and easy reduction in size of
the particles, and thus the metal particles can be easily dispersed in a
negative electrode mixture layer. In addition, the atomization method
eliminates the need for a grinding step.

[0047] The metal particles are more preferably formed by a gas atomization
method using inert gas. The gas atomization method using inert gas can
suppress the formation of oxides such as zinc oxide or aluminum oxide on
the surfaces of the particles and can form spherical metal particles.
Therefore, the specific surface per unit volume can be decreased.
Further, the metal particles can be uniformly dispersed in a matrix of
the carbon material, thereby reducing the stress produced in an electrode
due to a difference in expansion/contraction from the carbon material
mixed, such as graphite, during charge and discharge. Therefore, the
electrode structure can be stably maintained in repetition of charging
and discharging, and cycling life characteristics can be improved.

<Carbon Material>

[0048] Examples of the carbon material used in the present invention
include graphite, petroleum coke, coal-derived coke, carbides of
petroleum pitch, carbides of coal-derived pitch, phenol resins, carbides
of crystalline cellulose resins and carbon produced by partial
carbonization of the carbides, furnace black, acetylene black,
pitch-based carbon fibers, PAN-based carbon fibers, and the like. From
the viewpoint of conductivity and capacity density, graphite is
preferably used.

[0049] The graphite preferably has a crystal lattice constant of 0.337 nm
or less and as high crystallinity as possible because the conductivity
and capacity density are high, and the action potential is decreased,
thereby increasing the action voltage as a battery.

[0050] When the carbon material has a lame particle diameter, contact with
the metal is decreased, and conductivity on the negative electrode is
decreased. On the other hand, when the particle diameter is excessively
small, the specific surface is increased to increase the number of
inactive sites, thereby decreasing the efficiency of charge/discharge.
Therefore, in an embodiment of the present invention, the average
particle diameter of the carbon material is preferably in the range of
0.1 μm to 30 μm and more preferably in the range of 1 μm to 30
μm.

<Mixing of Metal Particles and Carbon Material>

[0051] With respect to the mixing ratio of the metal particles to the
carbon material, the ratio of the metal particles to the total of the
metal particles and the carbon material is preferably in the range of 1
to 60% by mass, more preferably in the range of 10 to 50% by mass.

[0052] In the use of a mixture of the metal particles and the carbon
material as the negative electrode active material, even when the packing
density of the negative electrode is increased, partial spaces are formed
between the metal particles and the carbon material, thereby improving
nonaqueous electrolyte permeability. That is, when the mixture of the
metal particles and the carbon material is used, lithium alloys with the
metal particles to cause a proper degree of expansion and contraction
during initial charge, and thus cracks, i.e., electrolytic solution
paths, can be formed in the negative electrode. Therefore, the nonaqueous
electrolyte permeability is improved. As a result, a nonaqueous
electrolyte secondary battery having a high capacity, a high energy
density, and excellent charge/discharge cycling properties can be
produced.

[0053] When the content of the metal particles is excessively small, the
effect of mixing with the metal particles may not be sufficiently
obtained. When the content of the metal particles is excessively large,
excessive growth of cracks or breakage of the negative electrode
structure may occur.

[0054] In order to uniformly disperse the metal particles in the negative
electrode mixture, the metal particles and the carbon material are
mechanically mixed using a stirring device or a kneading device such as a
mortar, a ball mill, a mechanofusion, or a jet mill.

<Polysaccharide Polymer>

[0055] In the present invention, the aqueous negative electrode mixture
slurry is prepared. A thickener suitable for aqueous slurry is used. In
the present invention, the polysaccharide polymer is used as the
thickener.

[0056] Examples of the polysaccharide polymer include carboxymethyl
cellulose compounds, cellulose compounds, amylose compounds, amylopectin
compounds, and the like. In particular, the carboxymethyl cellulose
compounds are preferred because of the excellent thickening properties.

[0057] The content of the polysaccharide polymer in the negative electrode
mixture slurry is appropriately controlled according to the types, the
contents etc. of the metal particles and the carbon material.

[0058] Carboxymethyl cellulose sodium salt (hereinafter, referred to as
"CMC") as a polysaccharide polymer may be used as a mixture with
styrene-butadiene rubber emulsion (hereinafter, referred to as "SBR") as
a binder.

<pH Adjustment>

[0059] In the present invention, the pH of the aqueous negative electrode
mixture slurry containing the metal particles, the carbon material, and
the polysaccharide polymer is adjusted in the range of 6.0 to 9.0. The pH
adjustment method is not particularly limited, but a method of adding a
pH buffer component to the negative electrode mixture slurry is
preferably used.

[0060] Examples of the pH buffer component include a phosphate buffer
component, a pH buffer component using tris(hydroxymethyl)methylamine,
and a pH buffer component using citric acid. In the present invention,
the phosphate buffer component is preferably used.

[0062] The content of the pH buffer component in the negative electrode
mixture slurry is appropriately adjusted so that the pH of the negative
electrode mixture slurry is in the range of 6.0 to 9.0.

<Preparation of Negative Electrode Mixture Slurry>

[0063] The negative electrode mixture slurry used in the present invention
contains the metal particles, the carbon material, and the polysaccharide
polymer, and is adjusted in the pH range of 6.0 to 9.0. As described
above, the pH is adjusted in the range of 6.0 to 9.0 by adding the pH
buffer component. In this case, the pH buffer component is preferably
contained in the negative electrode mixture slurry containing the
polysaccharide polymer before the metal particles are added to the
negative electrode mixture slurry. When the pH buffer component is
contained in the negative electrode mixture slurry before the metal
particles are added, an increase in pH can be suppressed when the metal
particles are added to the slurry. That is, the metal particles used in
the present invention have a higher ionization tendency than hydrogen,
and thus when the metal particles are added to the slurry containing
water as a dispersant, the metal particles react with water to generate
hydrogen and increase the pH of the slurry. An increase in pH of the
slurry causes the occurrence of aggregates due to aggregation of the
polysaccharide polymer. According to an embodiment of the present
invention, the occurrence of aggregate slurry can be efficiently
suppressed by suppressing an increase in pH of the slurry.

<Formation of Negative Electrode>

[0064] In an embodiment of the present invention, the negative electrode
can be formed by applying the negative electrode mixture slurry prepared
as described above to a current collector, for example, one including a
copper foil and then drying the slurry.

[0065] Further, after drying, the negative electrode is preferably rolled
with a rolling roller.

[0066] The packing density of the negative electrode is preferably 1.7
g/cm3 or more, more preferably 1.8 g/cm3 or more, and still
more preferably 1.9 g/cm3 or more. By increasing the packing density
of the negative electrode, the negative electrode having a high capacity
and high energy density can be formed. According an embodiment of to the
present invention, even when the packing density of the negative
electrode is increased, good charge/discharge cycling characteristics can
be achieved because of the excellent nonaqueous electrolyte permeability.

[0067] The upper limit of the packing density of the negative electrode is
not particularly limited, but is preferably 3.0 g/cm3 or less.

[0069] In order to increase the capacity density of the battery by
combining the positive electrode with the negative electrode, it is
preferred to use, as the positive electrode active material of the
positive electrode, a lithium-cobalt composite oxide containing cobalt
with a high action potential, for example, lithium cobaltate LiCoO2,
a lithium-nickel-cobalt composite oxide, a
lithium-nickel-cobalt-manganese composite oxide, a
lithium-manganese-cobalt composite oxide, or a mixture thereof. In order
to produce the battery having a high capacity, a lithium-nickel-cobalt
composite oxide or a lithium-nickel-cobalt-manganese composite oxide is
more preferably used.

[0070] The material for a positive electrode current collector on the
positive electrode is not particularly limited as long as it is a
conductive material. For example, aluminum, stainless, and titanium can
be used. In addition, for example, acetylene black, graphite, and carbon
black can be used as the conductive material, and for example,
polyvinylidene fluoride, polytetrafluoroethylene, EPDM, SBR, NBR, and
fluorocarbon rubber can be used as the binder.

[Nonaqueous Electrolyte]

[0071] As the nonaqueous electrolyte used in the present invention,
nonaqueous electrolytes generally used for nonaqueous electrolyte
secondary batteries can be used. For example, a nonaqueous electrolytic
solution containing a solute dissolved in a nonaqueous solvent and a gel
polymer electrolyte produced by impregnating a polymer electrolyte, such
as polyethylene oxide or polyacrylonitrile, with the nonaqueous
electrolytic solution can be used.

[0072] As the nonaqueous solvent, nonaqueous solvents generally used for
nonaqueous electrolyte secondary batteries can be used. For example,
cyclic carbonate and chain carbonate can be used. Examples of the cyclic
carbonate which can be used include ethylene carbonate, propylene
carbonate, butylene carbonate, vinylene carbonate, and fluorine
derivatives thereof. Preferably, ethylene carbonate or fluoroethylene
carbonate is used. Examples which can be used as the chain carbonate
include dimethyl carbonate, methylethyl carbonate, diethyl carbonate, and
fluorine derivatives thereof. Also, a mixed solvent prepared by mixing
two or more nonaqueous solvents can be used. A mixed solvent containing
cyclic carbonate and chain carbonate is preferably used. In particular,
when the negative electrode including the negative electrode mixture with
a high packing density is used, a mixed solvent containing cyclic
carbonate at a mixing ratio of 35% by volume or less is preferably used
for increasing permeability to the negative electrode. Further, a mixed
solvent containing the cyclic carbonate and an ether solvent, such as
1,2-dimethoxyethane or 1,2-diethoxyethane, can be preferably used.

[0073] Also, as the solute, solutes generally used for nonaqueous
electrolyte secondary batteries can be used. For example, LiPF6,
LiBF4, LiCF3SO3, LiN(CF3SO2)2,
LiN(C2F5SO2)2,
LiN(CF3SO2)(C4F9SO2),
LiC(CF3SO2)3, LiC(C2F5SO2)3,
LiClO4, Li2B10Cl10, Li2B12Cl12, and
the like can be used alone or in combination of two or more.

EXAMPLES

[0074] The present invention is described below with reference to
examples, but the present invention is not limited to these examples.

Example 1

[0075] Zinc spherical particles (manufactured by Kishida Chemical Co.,
Ltd., special grade, part No. 000-87575) having an average particle
diameter of 4.5 μm and produced by the atomization method were used as
a first active material. FIG. 1 shows a SEM (Scanning Electron
Microscope) image of the zinc particles used.

[0076] Artificial graphite having an average particle diameter of 22 μm
and a crystal lattice constant of 0.3362 nm was used as a second active
material.

[0077] The average particle diameters of the zinc particles and the
artificial graphite were measured using a laser diffraction particle size
distribution analyzer (SALAD-2000 manufactured by Shimadzu Corporation).

[0078] The first active material and the second active material were mixed
at a mass ratio (first active material:second active material) of 10:90.

[0080] The mixture of the first active material and the second active
material at the above-described mixing ratio was mixed with a
styrene-butadiene rubber (SBR) emulsion (solid content 48.5% by mass) at
a mass ratio of 97.5:1.5 to prepare a dispersion solution. The mixed
solution prepared as described above was mixed with the resultant
dispersion solution so that the mass ratio of (total of the first active
material and the second active material:CMC:SBR) was 97.5:1.0:1.5, and
the resultant mixture was kneaded to prepare a negative electrode mixture
slurry.

[0081] The pH buffer component was added in an amount of 0.5 g relative to
1 g of the slurry solid content (active materials, CMC, and SBR). The
measured pH of the negative electrode mixture slurry is shown in Table 1.

[0082] Next, the negative electrode mixture slurry was applied to a
negative electrode current collector including a copper foil, dried at
80° C., and then rolled with a rolling roller. Then, a current
collector tab was attached to form a negative electrode.

[Measurement of Number of Aggregates in Electrode]

[0083] The number of aggregates having a diameter of 1 mm or more was
measured by observing the surface of the resultant negative electrode.
The number of aggregates per 10 cm2 is shown in Table 1.

<Formation of Test Cell>

[0084] A test cell shown in FIG. 2 was formed using the negative
electrode. In a glove box under an argon atmosphere, the test cell was
formed using the negative electrode as a working electrode 1 and a
lithium metal for each of a counter electrode 2 and a reference electrode
3. An electrode tab 7 was attached to each of the working electrode 1,
the counter electrode 2, and the reference electrode 3. The working
electrode 1, the counter electrode 2, and the reference electrode 3 with
polyethylene separators provided between the working electrode 1 and the
counter electrode 2 and between the counter electrode 2 and the reference
electrode 3 were sealed, together with a nonaqueous electrolytic solution
5, in a laminate container 6 composed of an aluminum laminate, thereby
forming test cell A1.

[0085] The nonaqueous electrolytic solution 5 used was prepared by
dissolving lithium hexafluorophosphate (LiPF6) at a concentration of
1 mol/liter in a mixed solvent containing ethylene carbonate and
ethylmethyl carbonate at a volume ratio of 3:7.

[0086] The test cell formed as described above was, at room temperature,
charged until the potential reached 0 V (vs. Li/Li.sup.+) with a constant
current of 0.2 mA/cm2 and then discharged until the potential
reached 1.0 V (vs. Li/Li.sup.+) with a constant current of 0.2
mA/cm2. The initial discharge capacity at the 1st cycle and the
discharge capacity at the 5th cycle after repetition of the
charge/discharge cycle were determined. The results are shown in Table 1.

Example 2

[0087] A negative electrode was formed by the same method as in Example 1
except that the mixing ratio of the buffer component was 1.0 g relative
to 1 g of the slurry solid content, and test cell A2 was formed using the
formed negative electrode.

[0088] The pH of the negative electrode mixture slurry, the number of
aggregates in the electrode, the initial discharge capacity, and the
discharge capacity at the 5th cycle were measured. The results are shown
in Table 1.

Comparative Example 1

[0089] A negative electrode was formed by the same method as in Example 1
except that the pH buffer component was not mixed when the negative
electrode mixture slurry was prepared, and test cell X1 was formed using
the formed negative electrode.

[0090] The pH of the negative electrode mixture slurry, the number of
aggregates in the electrode, the initial discharge capacity, and the
discharge capacity at the 5th cycle were measured. The results are shown
in Table 1. In Table 1, the amount of the pH buffer component mixed
represents the ratio by mass of the pH buffer component to the solid
content in the negative electrode mixture slurry.

[0091] Table 1 indicates that in Comparative Example 1 in which the pH
buffer component was not added to the negative electrode mixture slurry,
the pH of the negative electrode mixture slurry is 11.16. On the other
hand, in Examples 1 and 2 in which the pH buffer component was added to
the negative electrode mixture slurry, the pHs of the negative electrode
mixture slurries are 7.88 and 7.47, respectively. In Examples 1 and 2 in
which the pH of the negative electrode mixture slurry was adjusted in the
range of 6.0 to 9.0 according to the present invention, as shown in Table
1, the number of aggregates in the electrode is 0. While in Comparative
Example 1, the number of aggregates is more than 100.

[0092] Therefore, it is found that when the pH of the negative electrode
slurry is adjusted in the range of 6.0 to 9.0 according to the present
invention, an increase in pH can be suppressed when zinc particles are
added, and thus aggregation of the polysaccharide polymer due to an
increase in pH can be suppressed.

[0093] Table 1 also indicates that in Examples 1 and 2, the initial
discharge capacity and the discharge capacity at the 5th cycle are more
improved than in Comparative Example 1. Therefore, it is found that when
the pH of the negative electrode slurry is adjusted in the range of 6.0
to 9.0 according to the present invention, the occurrence of aggregates
can be suppressed when the negative electrode is formed, and thus a
nonaqueous electrolyte secondary battery having a high capacity, a high
energy density, and excellent charge/discharge cycling characteristics
can be produced.

<SEM Observation of Surface of Negative Electrode>

[0094] The surfaces of the negative electrodes formed in Examples 1 and 2
and Comparative Example 1 were observed with SEM. FIGS. 3, 5, and 7 show
5000-times magnified SEM images of the surfaces of the negative
electrodes formed in Examples 1 and 2 and Comparative Example 1,
respectively. FIGS. 4, 6, and 8 show 5000-times magnified SEM reflection
electron images of the surfaces of the negative electrodes formed in
Examples 1 and 2 and Comparative Example 1, respectively. In each of the
SEM reflection electron images, zinc particles are shown in white, and
graphite particles are shown in black.

[0095] FIGS. 3 to 8 indicate that in Comparative Example 1, which does not
contain the buffer component, the zinc particles and the graphite
particles form aggregates, while in Examples 1 and 2, containing the pH
buffer component according to the present invention, no aggregate is
observed.

Example 3

[0096] A negative electrode was formed by the same method as in Example 1
except that a pH standard solution (Kishida Chemical Co., Ltd.) including
an aqueous solution containing 0.36% by mass of disodium hydrogen
phosphate (Na2HPO4) and 0.68% by mass of potassium dihydrogen
phosphate (KH2PO4) was used as the pH buffer component and
mixed in an amount of 1.0 g relative to 1 g of the solid content in the
negative electrode mixture slurry.

[0097] The pH of the negative electrode mixture slurry and the number of
aggregates in the electrode were measured by the same method as in
Example 1. The results are shown in Table 2.

[0098] Table 2 indicates that in Example 3 in which the pH buffer
component was added to the negative electrode mixture slurry, the pH of
the negative electrode mixture slurry is 8.50, and the number of
aggregates in the electrode is 0. In contrast, in Comparative Example 1
in which the pH buffer component was not added to the negative electrode
mixture slurry, the pH of the negative electrode mixture slurry is 11.16,
and the number of aggregates in the electrode is more than 100.

[0099] These results indicate that when the pH of the negative electrode
mixture slurry is adjusted in the range of 6.0 to 9.0 according to the
present invention, the occurrence of aggregation of the polysaccharide
polymer and aggregates of the metal particles and the carbon material in
the negative electrode can be suppressed. The aggregates of the metal
particles and the carbon material are considered to be produced by
aggregation of the polysaccharide polymer. According to the present
invention, an increase in pH can be suppressed when the metal particles
are added to the negative electrode mixture slurry, and thus aggregation
of the polysaccharide polymer can be suppressed, thereby suppressing the
occurrence of aggregation of the metal particles and the carbon material
due to aggregation of the polysaccharide polymer. By suppressing
aggregation of the metal particles and the carbon material, a nonaqueous
electrolyte secondary battery having a high capacity, a high energy
density, and excellent cycling characteristics can be produced.

[0100] In each of the examples, the negative electrode formed by the
production method of the present invention was evaluated by forming the
test cell using metallic lithium for the counter electrode, and. However,
even when the negative electrode is incorporated as a negative electrode
for a nonaqueous electrolyte secondary battery, the same results can be
obtained.

[0101] While detailed embodiments have been used to illustrate the present
invention, to those skilled in the art, however, it will be apparent from
the foregoing disclosure that various changes and modifications can be
made therein without departing from the spirit and scope of the
invention. Furthermore, the foregoing description of the embodiments
according to the present invention is provided for illustration only, and
is not intended to limit the invention.